The present application relates to a reproducing apparatus and a reproducing method for reproducing data from a hologram recording medium on which data in predetermined page units is recorded by interference fringes of reference light and signal light. The present application relates to a recording and reproducing apparatus and a recording and reproducing method for recording data in and reproducing data from such a hologram recording medium. Further, the present application relates to a recording apparatus and a recording method for recording data on such a hologram recording medium.
In a hologram recording and reproducing system, in particular, a hologram recording and reproducing system in the field of optical storage systems, an SLM (Spatial Light Modulator) such as a transmission liquid crystal panel or a DMD (Digital Micro mirror Device) is used for light intensity modulation. The SLM applies intensity modulation for obtaining a pattern array of bit 1 (e.g., light intensity is high) and bit 0 (e.g., light intensity is low) to signal light.
For example, as shown in FIG. 2, the SLM applies light intensity modulation to light in the center thereof according to recording data to generate signal light and transmits the light in a ring shape around the signal light to generate reference light. The signal light modulated according to the recording data is irradiated on a hologram recording medium together with the reference light. Consequently, interference fringes of the signal light and the reference light are recorded on the hologram recording medium.
During reproduction of data, the SLM generates only the reference light and irradiates the reference light on the hologram recording medium to obtain diffractive light corresponding to the interference fringes. An image corresponding to the diffractive light is focused on an image sensor such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor to obtain amplitude values of respective recording pixels (i.e., respective pixels of the SLM). Reproduced data is obtained on the basis of the amplitude values of the respective pixels.
The hologram recording and reproducing system for irradiating the signal light and the reference light on an identical optical axis in this way is known as a coaxial system. When the coaxial system is adopted, it is extremely difficult to strictly set respective pixels of the SLM and respective pixels of the image sensor in a one to one relation because of optical distortion, a magnification, and the like. In other words, it is extremely difficult to precisely make reproduced light corresponding to the respective pixels of the SLM incident on assumed respective pixels on the image sensor.
Thus, during recording, in arraying data in signal light, predetermined pattern data called syncs are inserted at predetermined intervals and, during reproduction, alignment based on positions of the syncs is performed and, then, calculation of amplitude values of the respective pixels is performed.
By inserting the syncs in this way and aligning the positions of the syncs and positions where patterns of the syncs are actually detected, it is possible to correct deviation of reproduced light from an ideal incidence position due to optical distortion and the like and realize an appropriate readout operation.
However, the deviation of an incidence position does not always occur in a unit of one pixel. It is anticipated that the deviation occurs in a unit smaller than a pixel as well. When the deviation in a unit smaller than a pixel occurs, it is difficult to perform accurate detection of the syncs for performing alignment. This makes it difficult to perform not only data readout but also alignment.
Therefore, to make it possible to cope with such deviation in a unit smaller than a pixel, for example, the number of pixels of the image sensor is set to n time (at least 4 (2×2) times) as large as the number of pixels to improve a resolution of the image sensor for reproduced light for one pixel of the SLM.
For example, in a general method in the past, the reproduced light for one pixel of the SLM is received by 4 (2×2) pixels on the image sensor (2×2 over-sampling). Consequently, it is possible to obtain a resolution of the image sensor four times as high as that of one pixel of the SLM.
In the technique in the past, in order to further improve accuracy, for example, respective values obtained by the 2×2 over-sampling are interpolated to further perform 2×2 up-convert (up-convert to a pixel size of 4×4). In other words, by performing the up-convert to a pixel size of 4×4, it is possible to increase the resolution of the image sensor to be sixteen times as high as that of one pixel of the SLM.
For example, it is possible to perform correction of readout positions of the respective pixels by a unit smaller than a pixel according to such a method. Thus, it is possible to appropriately perform calculation of amplitude values of the pixels.
When it is assumed that each of the pixels of the image sensor outputs a reception signal level in, for example, gradations of 0 to 255, an amplitude value corresponding to a bit value “0” is, for example, about “64” and an amplitude value corresponding to a bit value “1” is, for example, about “192”.
Therefore, if an amplitude value of a position of each of the pixels specified after the positional correction with the syncs (in this case, sixteen values after the over-sampling and up-convert correspond to one pixel of the SLM) is about “64”, a bit value of the position can be judged as “0”. If the amplitude value is about “192”, a bit value of the position can be judged as “1”.
By performing such detection of a position of a pixel and calculation of an amplitude value of the detected pixel position for all pixel positions, it is possible to reproduce data recorded on the hologram recording medium as a modulation pattern in the SLM.
FIG. 23 and FIGS. 24A and 24B are diagrams for explaining an example of a specific recording data format adopted in the hologram recording and reproducing method in the past.
FIG. 23 schematically shows a modulation pattern in the SLM. FIGS. 24A and 24B show examples of patterns of syncs inserted in recording data.
As it is understood from the above explanation, in the case of the coaxial system, it is necessary to irradiate reference light during recording. Thus, as shown in FIG. 23, a reference light area for generating this reference light is defined in an outermost peripheral portion of the SLM.
In an inner peripheral side portion of this reference light area, a signal light area in which a pattern recorded on the hologram recording medium should actually be formed is defined across a gap area shown in the figure.
During recording, a data pattern in this signal light area is sequentially changed and data recording on the hologram recording medium is performed. A volume of data recorded at a time by interference with the reference light is a volume of data that is laid in the signal light area. A unit of data recording performed by interference at a time in this way (i.e., a unit of a volume of data laid in the signal light area) is called “page”.
Data units other than this “page” are defined. For example, 4 bits×4 bits is called “symbol”. Besides this “symbol”, 6 symbols×6 symbols (i.e., 24 bits×24 bits) is called “sub-page”.
In FIG. 23, in the technique in the past, in forming data as one page, data is laid in the circular signal light area with one sub-page as a minimum unit as shown in the figure.
In laying the data, syncs are inserted at predetermined intervals. In the technique in the past, a page sync shown in the figure is inserted at the top (in this case, a position at the left end at the uppermost stage) of each page.
This page sync is a sync used for performing rough alignment of entire one page.
For the page sync, as shown in FIG. 24A, one sub-page is allocated. As a data pattern of the sub-page, as shown in the figure, a pattern in which all bits of 4 bits×4 bits (bits for 1 symbol) in the center of 4 (2×2) symbols located in the center of one sub-page are set as “1” and all bits other than the bits are set as “0” is defined. In this case, data patterns for the 4 symbols in the center are as follows:    “00000000    00000000    00111100    00111100    00111100    00000000    00000000”.
All the other bits in the sub-page are “0”.
Moreover, in one page, a sub-page sync shown in FIG. 24B is given to each sub-page set as a minimum unit of data laying as described above.
As this sub-page sync, a pattern same as the 4 symbols in the center of the page sync described above is inserted with respect to the 4 symbols in the center in one sub-page.
This sub-page sync is used for final alignment during calculation of an amplitude value of a pixel. When an amplitude value of each pixel is calculated in each sub-page, a position of an object pixel is specified with a position of the sub-page sync as a reference, an amplitude value of the specified pixel position is calculated, and, then, the data is identified as a final bit value of “0” or “1”.
In this way, in the format in the past, the data is laid in the signal light area with one sub-page (24 bits×24 bits) as a minimum laying unit. Concerning the syncs, the page sync (for one sub-page) is inserted for each page and the sub-page sync (for 4 symbols) is inserted for each sub-page in one page.
With such a format, in the technique in the past, it is possible to pack user data (data other than the syncs) for 3552 symbols as the number of effective symbols in a signal light area with a radius of 154 pixels (pixel is one pixel of the SLM) as shown in FIG. 23. In other words, an effective capacity for one page is 3552 symbols.
As one of techniques in the past related to the present application, there is JP-A-2006-196044.